Lignin is a natural macromolecule in the cell wall of vascular plant. It is a naturally existing amorphous biopolymer widely found at an average of 20-30% dry mass and is the second most abundant organic polymer on Earth. Lignin contains hydroxy- and methyoxy-substituted phenylpropane units. It is also a high-volume by-product of wood pulping, which exceeds 15 million tons per year in the United States. Lignin must be removed before wood is turned into high quality paper and the three dimensional network of lignin in wood is generally disrupted under alkaline conditions. Newsprint, brown sack paper, and cardboard each contain certain levels of lignin.
There are a number of types of lignin. Sulfite pulping yields lignosulfonates (lignin sulfonates or sulfite lignins). Alcell lignin comes from the organosolv process and contains very small amounts of inorganic materials. Alkali lignin (also known as Kraft lignin or sulfate lignin) is the dominant lignin by-product from the Kraft pulping process, the predominant wood pulping process today. The Kraft process is the conversion of wood into wood pulp consisting of almost pure cellulose. Wood chips are first treated with a mixture of sodium hydroxide and sodium sulfide, which break the bonds that link lignin to cellulose. Lignin is then isolated from the resulting black liquor with alkali and precipitated using mineral acids. Alkali lignin thus contains high amount of inorganic materials (ashes and salts). Based on the type of wood and the extraction process used, the physical and chemical properties of lignin can differ.
To date lignin has been used mostly as a low-grade fuel, providing heat and power to run mills and associated wood pulping processes. However, there is a growing demand to identify applications with high economic value for the lignin by-product of wood pulping.
Carbon fibers have excellent chemical, electrical, magnetic and mechanical properties and thus have a wide range of potential technical applications. One representative application of carbon fibers is so-called high performance fibers, which generally have superior mechanical properties useful for fiber-reinforced composite materials; and a second example is general purpose carbon fibers, which generally have high specific surface areas. These high surface area carbon fibers typically have applications that do not rely on their mechanical properties. Such applications include, but are not limited to, catalysis, adsorption/separation, energy storage and conversion, gas storage, nanoelectronics and other application requiring materials with a high specific surface area.
Preparation of carbon fibers from various types of lignin has been suggested; but, a number of problems, including but not limited to low mechanical properties of the resulting carbon fibers, have prevented their commercial use in reinforcement and/or composites applications. In one approach, carbon fibers have been generated using melt extrusion. The diameter of the resulting carbon fibers generally ranges from 30-80 microns and carbon fibers have not been successfully prepared at smaller diameters because of limitations in the melt extrusion process.
The rapidly developing technology of “electrospinning” provides a mechanism to produce nano-scaled polymer fibers (generally with diameters <1,000 nm). Electrospinning low-cost and renewable alkali lignin into carbonaceous nanofibrous materials would provide a novel method of manufacture for a product with many applications.
Energy storage and conversion has become an important global subject, including research efforts on fuel cells, batteries and capacitors. Electrochemical capacitors are important electrical energy storage devices. New type of electrochemical capacitors that have specific capacitance values up to 10,000 times of electrolytic capacitors have been developed rapidly in recent years. In general, two energy storage modes are present in these super high capacitance capacitors: electric double layer capacitors (also referred to as supercapacitors or ultracapacitors) and pseudocapacitors. Supercapacitors store and release electrical energy by ion absorption and desorption on electrode surface and their capacitance is generally proportional to the specific surface area of their electrodes. Pseudocapacitors achieve energy storage and release by charge transfer at electrode surface between electrode and electrolyte via reversible redox or Faradaic reactions.
Supercapacitors have demonstrated application in memory backup system, auxiliary power unit, instantaneous power compensation, and energy storage. Electrode material is one factor that influences the efficiency and practicality of supercapacitors. To date, a variety of carbon materials have been investigated as electrode materials for supercapacitors such as traditional activated carbon, carbon nanotubes, carbon nanofibers, carbon aerogels, carbide-derived carbon, and composite materials containing vanadium oxide and graphene nanosheets. Among current electrode materials, activated carbon is one that is widely used. However, the process of activating carbon is costly and energy-consuming. In addition, preparation of a free-standing electrode from activated carbon generally requires additional organic binder, which can degrade the overall performance of the electrode.
The generation of free-standing carbon electrode materials with high specific surface area from renewable carbonaceous sources remains a challenge for high performance supercapacitors.